Methods for controlling the angular rotor speed of induction motors have evolved from variable frequency control to vector control and other variants, e.g., direct and indirect field oriented state feedback control, sensorless speed control, and adaptive field oriented control. Vector control with a full state or rotor speed measurements can provide adequate performance at the expenses of additional sensors, which can limit their application in practice.
Sensorless speed control includes sensorless adaptive control, where the rotor speed is typically treated as an unknown parameter to avoid nonlinearity in dynamics. Controls relying on parameter assumptions generally have unsatisfactory transient performance inherent to the adaptation.
Other methods that use a high gain state estimator, sliding mode estimator, or an extended Kalman filter (EKF) avoid the parameter assumptions, but fail to address performance. As an example, resorting to nonlinear estimator techniques entails a system in certain normal forms, which turns out to be difficult. Well-known high gain estimators designs assume an observable canonical form.
FIG. 1 shows a prior art sensorless speed motor drive for an induction motor. The reference numerals for FIG. 1 are:                101 Speed control        102 Flux control        103 Current control        104 Clarke Transformation/Power electronics        105 Induction motor        106 Flux estimator        107 Speed estimator        111 desired speed        112 estimated speed        113 estimated speed error        114 desired q-axis current        115 estimated/measured q-axis current        116 q-axis current tracking error        117 desired flux modulus        118 estimated flux modulus        119 estimated flux error        120 desired d-axis current        121 estimated/measured d-axis current        122 d-axis current tracking error        123 desired voltage input commands        124 measured voltage inputs        125 current measurements        126 estimated flux.        
Input to the motor drive is a reference rotor flux amplitude signal 111. An estimate 112 from a flux estimator block 106 is added to the signal 111 so that the signal 113 represents a difference between signals 111 and 112.
A flux control block 101 determines a stator current 114 used to control the rotor flux linkage in the d-axis. A signal 115 is an estimate or true stator current, in the d-axis, produced by a flux estimator 106. A difference 116 between the signals 115 and 114 is used by a current control block 103 to determine a reference stator voltage 123 in the d-axis. Similarly, a signal 117 denotes the desired rotor speed reference of the induction motor.
A signal 118 denotes an estimated rotor speed produced by a speed estimator 107 based on output signal 126 of the flux estimator 106. A difference 119 between signals 117 and 118 is used to determine a reference stator current 120, in the q-axis, by the flux control block 102.
An estimated or true stator current 121, in the q-axis, is compared to the reference stator current 120, in an imaginary q-axis used to control the torque of the motor, to produce a difference signal 122. The current control block 103 determines the stator voltage signal 123, in d- and q-axes, on the basis of difference signals 116 and 122. A Clarke or Park transformation 104 converts the desired stator voltage, in d- and q-axes, into three-phase voltages 124 to drive the induction motor 105.
Note that the flux estimator 106 takes the three-phase voltages 124 and sensed phase currents 125 as input signals, and outputs estimated or true stator currents 115 and 121, estimated rotor flux amplitude 112, and estimated rotor speed signal 118 to produce the difference signals 113, 116, 119, and 122. The estimated stator current and the estimated rotor flux amplitude, and the estimated rotor speed can be determined independently. The signal 119 is used for speed control 102.